I found a bottoming tap that would just start to bite into the cavity; then used a short spacer, washer and nut to form an ad-hoc puller. (Turning the nut on the tap to pull it from the hole.) They slipped right out, and could have been re-used. But instead, I opted to thread the holes and make up threaded plugs like the earlier covers use.
Sorry I don't recall the threads offhand, but they were standard SAE sizes. The holes were the perfect size to tap for plugs, no drill involved. To make the plugs, I just cut the tip off a standard bolt with a Dremel & cutoff wheel, then used the same tool to form a screwdriver slot.